1. Field of the Invention
The present invention relates to a control device for an electric motor. More in detail, this invention relates to a sensor-less control technique for a three-phase synchronous motor equipped with permanent magnets for a rotor, which will be called as “internal permanent magnet (IPM) motor” hereinafter.
2. Description of Related Art
In order to control the rotating speed of an electric motor to be driven by an inverter, it is necessary to detect the positions (phase) of poles of a rotor. As a method of detecting the position of a rotor without any sensor, there is an article titled “Evaluation for estimated accuracy in the positional sensor-less field detecting method for PM (permanent magnet) motor, using current vector locus” (No. 683, at National Convention in 1994 of Society for Industrial Application, Institute of Electric Engineers). The article describes a technique to detect a long axis of current vector locus in a high-frequency space by superimposing a high-frequency current on a drive current, thereby detecting the phase of a rotor.